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Reflections on ULAS J1120+0641 and GN-z11
In article ,
jacobnavia writes: https://arxiv.org/pdf/1603.00461.pdf Published in ApJ at http://dx.doi.org/10.3847/0004-637X/819/2/129 There, we can read that this galaxy is very bright (across more than 13 billion years imagine). How they estimated the stellar population? I went to Appendix B, "Estimating stellar population properties" and there, I could read: Answering your own question there. quote The allowed star-formation histories were based on standard exponentially declining models with parameters tau = 108, 109, and constant star-formation, with stellar metallicities of Z = 0.05 - 0.57 Zo and an initial Chabrier mass function end quote [Here Zo is a Z followed by a circle with a dot. I assume here that that is the metallicity of the sun. Please correct me if I am wrong.] That last is correct; circle with dot is the symbol for the Sun. But wait a minute here. We are just a few hundred million years after a big "bang"... Can we apply any models based on data MUCH later in the history of the universe? The stellar evolution tracks are probably OK -- why wouldn't they be? The initial mass function (IMF) is strictly an assumption, but one has to assume something. For both local galaxies and distant ones, the observed _light_ comes from the massive stars. In local stellar populations, the _mass_ is mostly in the low-mass stars. The "Chabrier" IMF assumption is typical, but there is no direct evidence that any low-mass stars at all are present. In that sense, the derived stellar mass is more or less an upper limit. By definition this galaxy shouldn't contain much "metals" Why not? A "generation" of massive stars takes only about 10 Myr to produce supernovae, which will pollute the interstellar medium with metals. The zero-metallicity IMF is likely to be top-heavy, so a large fraction of the mass making up the first generation of stars is likely to be recycled into metal-rich gas. How can we have any metallicity or dust at all? The dust extinction actually derived from the data was an upper limit, i.e., consistent with zero, so you should be happy enough with the result. And besides that, the problem is that to make stars efficiently, galaxies need "metals" as seeds. Making stars without any metals is a MUCH SLOWER process... Why do you think that? It's harder to make low-mass stars, but I don't see why the process for high-mass stars should be slower. Stars form in VERY cold environments in galaxies, protected from radiation by dust and cool gas. They do _now_. That doesn't make it impossible for them to have formed in warmer environments in the past. The required mass would have been higher, of course. Look up "Jeans mass." Massive galaxies at large redshift were very rare (so far as we know now), but their abundance was (apparently) not strictly zero. The evidence should get much stronger when JWST starts working. -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
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